The role of endogenous porphyrins in the effects of laser radiation of the red region (632.8 nm) on free radical processes in the blood of rats under endotoxic shock induced by the administration of lipopolysaccharide B (25 mg/kg) has been studied. The measurements of the functional activity of polymorphonuclear leukocytes (the method of luminol-dependent chemiluminescence), the superoxide dismutase activity of blood plasma (using nitro blue tetrazolium), and the degree of lipid oxidation of erythrocyte membranes (the method of fluorescence of cis-parinaric acid) have been carried out. It has been found that low-intensity laser radiation strongly affects all processes examined irrespective of the administration of lipopolysaccharide B. The effect of radiation was most pronounced in animals injected with the polysaccharide, the changes being dependent on the concentration of endogenous porphyrins in samples.